Myriad approaches have been pursued to control pests. Many of these methods and compositions are directed to control of pests that attack plants, most notably commercially valuable plants. Although much current agricultural research has pest control as its objective, pest destruction of plants and plant products is still a major problem.
Pesticides, biological or chemical agents that kill pests, have been encapsulated in starch which was crosslinked with borate, calcium, or xanthide, thereby producing a matrix that can be processed into granules of desired sizes and densities (Shasha et al., 1984; Trimnell et al., 1982; Wing et al., 1983). However, these methods cannot be used for most living organisms because the reagents and conditions are too harsh for their survival.
Controlled release by means of starch-based encapsulating materials can also be accomplished without the use of chemical crosslinking reactions. In U.S. Pat. No. 2,876,160, Schoch et al. disclose such a method which employs modified, amylose-free starches at concentrations up to 65% solids for embedding water-insoluble materials.
In PCT Int. Appl. WO 85/04074, Flashinski et al. disclose two methods of preparing a starch gel matrix containing an insecticide. The insecticide is either coextruded with a dilute, aqueous dispersion of starch, or the starch is first partially cooked in an extruder prior to cold-blending with the insecticide. In either case, the product is recovered and used as an aqueous gel.
In U.S. Pat. No. 4,230,687, Sair et al. disclose the application of shearing stress, vigorous mechanical working, and heat to distribute an active agent into an enveloping matrix of chemically modified starches, gums, and proteins in the presence of a limited quantity of water. Proteins are used for slow-release matrices; modified starches are used for rapid release.
Similarly, in U.S. Pat. No. 3,922,354, Galuzzi et al. disclose the use of high-shear mixing to incorporate active agents into low-water, high-solid matrices prepared from partially gelatinized unmodified starches. Additives such as modified dextrins, mixtures of mono and diglycerides, toasted cereal solids, and coloring agents are used to control the release of active agents.
In U.S. Pat. No. 3,666,557, Jensen et al. disclose a method of using low-fat starchy materials to microencapsulate individual beadlets of sensitive materials such as vitamins and vegetable oils. Starches are prepared for encapsulation by heating at 88.degree. C. for 30 minutes followed by passage through a homogenizer to effect disruption of granules without degradation of molecules.
One general approach has been to develop carriers for insect control agents. Controlled-release systems based on entrapment of insect control agents in natural polymer matrices have some advantages. For example, hydrated hydrogel capsules have been used to incorporate insecticidally effective amounts of nematodes (U.S. Pat. No. 4,701,326). Capsules in that patent were designed to prevent desiccation. Other types of carriers have been granules.
Insecticides have been formulated on various granule carriers (Synek, 1983; Vander Hooven, 1983). Clay granules have been used to encapsulate entomopathogens (Raun et al., 1966). These carriers may be classified as baits which the insect must feed upon, or inert particles such as clay or corn cob which carry the active agent to the target site and then depend on environmental factors to release the active agent into the feeding zone. Baits have been used in control efforts against grasshoppers (Shotwell, 1944) and, more recently, against European corn borers (McGuire et al., 1990). Because baits must be ingested, they are much more specific than either liquid sprays, dusts, or inert granule carriers.
Integrated pest management programs against ground-dwelling insects such as the fire ant and certain grasshopper species that require selective pesticide usage are areas of application for these types of baits. However, currently available baits have limited utility for controlling most leaf-feeding pests because the granules are easily dislodged from the feeding zone, thus rendering them ineffective against the target pest. Additionally, theoretical calculations show that at application rates of 1 pound/acre, less than 0.01% of the leaf area is covered, assuming particle sizes of approximately 1 millimeter (mm) diameter (Koestler, 1980). To maximize the chances of an insect discovering the granule, volatile attractants may be incorporated into the formulation (e.g. Meinke et al. 1989; Metcalf and Lampmann, 1989; Lance & Sutter 1990).
The granule carriers for insect control agents have generally been suitable only for control of soil-borne pests. They have not been useful for foliar control of insects on plants because they have difficulties sticking to the foliage, or being consequently susceptible to removal by wind, rain, or other disturbing forces. Granule carriers for foliar insect control have neither been efficient nor economic.
Methods for encapsulating entomopathogens within starch matrices have been developed. (Dunkle and Shasha, 1988, and U.S. Pat. No. 4,859,377). Recently, a series of papers has examined the potential for using the Dunkle & Shasha (1988) techniques. One class of agents which were encapsulated within these starch matrices were bacteria, most notably Bacillus thuringiensis Berliner for control of European corn borers (McGuire et al., 1990). Grasshopper entomopoxvirus for control of rangeland grasshoppers was similarly formulated (McGuire, et al., 1991).
In another application the starch encapsulation process for preparing a formulation was used to control adult Diabrotica spp. (e.g., Lance and Sutter, 1990; Weissling & Meinke, 1991). Ingredients in the formulation included an attractant, a feeding stimulant (cucurbitacin), and a small amount of insecticide. Although preliminary results were promising, the formulation would not stay in the feeding zone. The development of new, adherent granules is required to aid in this bait approach. A significant problem is that traditional baits generally do not stick well to plant foliage and soon disappear from the feeding zone of the target insect, requiring constant and costly replenishment.
Pesticides were encapsulated by starch xanthate, but methods of production were undesirable due to flammable and toxic components. Other starch-based systems are reviewed by Trimnell and Shasha (1988). These required large numbers of steps placing severe limitations on their commercial use.
A method was developed by Trimnel and Shasha (1988) to form carrier starch granules using relatively small amounts of water. In the Trimnell and Shasha method, a pesticide was mixed with the pregelatinized starch or ungelatinized starch containing a gelatinizing agent and sufficient water to form granules. The sequence of steps in their method was first to mix a solution of chemical herbicide and an organic solvent, and subsequently to add water. By this method granules were formed that encapsulated the pesticide upon contact with the free water. However, these granules required further processing that limited their usefulness.
In addition to deficiencies in the composition of the starch granules, the methods of producing the granules with living entomopathogens, also have serious limitations which become more glaring the more scaled up the production. The basic method (Shasha and Dunkle U.S. Pat. No. 4,859,377) consists of adding water to modified starch at a proportion of at least 1:1 to produce a gelatinous mass. Unfortunately, when the laboratory procedures were scaled up to achieve mass production of the granules, the high water content caused difficulties. Heat could not be applied as a drying agent because entomopathogens will not survive the heat required to effect drying and, thus, would lose their effectiveness. The persistently high water content made grinding and drying of the gelatinous starch mass to produce granules not feasible for most commercial production. Simply reducing the water content resulted in uneven distributions of moisture in the mass.
In summary, despite numerous methods and compositions proposed for pest control, efficacy of granular, pest control agents applied to surfaces, for example, plant foliar surfaces, has not been attained.